This work validates the ability for in-band full-duplex systems to operate over ultra-wide bandwidths. As a proof of concept an ultra-wideband in-band full-duplex antenna array was designed. The array is formed of two wideband microstrip-slotline baluns and four Vivaldi antennas in a 3D-printed single-piece structure. The metal thickness of the 3D-printed Vivaldi antennas makes them sturdier than PCB antennas and more immune to fracturing or breaking, however, they require a coax-to-slot transition as a feeding mechanism. Such transitions are not well documented in the literature, specifically, the effect of metal thickness on the impedance of the 3D-printed slotline. Therefore, impedance curves for the 3D slot are provided as a function of its width and metal thickness. Then, a prototype was realized and measured, achieving about 40 to 80 dB of self-interference cancellation in the frequency range from 4.4 GHz to 28.7 GHz, and an average gain of 9.8 dBi with a directive end-fire radiation pattern. The only drawback of this first prototype is the presence of grating lobes in its radiation pattern due to the separation distance between the opposite antennas which exceeds a half-wavelength. Thus, to mitigate those grating lobes, we propose an approach based on a partial size reduction at the bottom of the antenna. This allows bringing the antennas closer together at their bottom parts, reducing the distance between the radiating slots, which translates to grating lobes reduction especially at higher frequencies. The implementation of this approach results in keeping all sidelobes 10 dB below the main lobe level, while keeping the Figure of Merit of this 2nd ultra-wideband in-band full-duplex antenna system at a higher level compared to existing techniques.